Replace problems

Well I am using the standard replace function in an aspx file, heres the code:

dim strpx as string
dim strbegin,strend,endvalue,intCounter,oldvalue as integer
strbegin = 5
oldvalue = 0
'for intCounter = 1 to 3
do
oldvalue = oldvalue + 1
response.write (strbegin & " <b>" & oldvalue & "</b><br>")

strbegin = inStr(strbegin,newimageurl,"top:") + 4


strend = inStr(strbegin,newimageurl,";") - strbegin

response.write (strend & "<br>")
response.write (instr(strbegin,mid(newimageURL,strbegin,strend),"px") & "<br>")
if strbegin <> 4 then
if len(mid(newimageURL,strbegin,strend)) > 3 then
endvalue = (mid(newimageurl,strbegin,strend - 2) + 150)
strpx = "px;"
else
endvalue = (mid(newimageurl,strbegin,strend) + 150)
strpx = ";"
end if

response.write (endvalue & "<br>" & (strbegin - 4) & "<br><br>")

newimageurl = replace(newimageurl,"top:" & mid(newimageurl,strbegin,strend) & ";","top:" & endvalue & strpx,(strbegin - 4),1)
end if

loop until strbegin < 5

the bit what is creating the problem is:

newimageurl = replace(newimageurl,"top:" & mid(newimageurl,strbegin,strend) & ";","top:" & endvalue & strpx,(strbegin - 4),1)

More specifically the start that I have set as a variable (strbegin - 4)

if I leave this as 1 it goes through and works fine on most pages, but on a few pages the numbers are duplicated and it modifies the same one twice and other ones none at all. when I try to set the start to the variable so it starts in the right place it ends up replacing most of the areas I am searching and replacing.

I have checked the variable and I am at a complete loss as to what is happening as it is definately the right number?

The only thing I can think of is if it is because of the .aspx (.net) extension not liking variables for the start area of the replace function.

Any help greatly appreciated.

never mind I figured out what was wrong, the start argument deletes everything in the string to the left of the character count